Plasma graft polymerization of acrylamide onto crosslinked HTPB based PU membrane for pervaporation

Pervaporation of water-ethanol mixtures through plasma graft polymerization of acrylamide onto crosslinked hydroxyl terminated poly-butadiene (HTPB) based PU membranes, plasma graft polymerization of acrylamide onto crosslinked PU membrane (AAm-p-CPU), were investigated. The grafting was dependent on the discharge power and pretreatment period. The effects of crosslinking, plasma treatment conditions, feed compositions, and feed temperature on the performance of these membranes were studied. The physical properties of crosslinked membrane were better than those of the uncrosslinked membrane. In addition, compared with crosslinked PU membranes (CPU), the plasma modified crosslinked PU membranes effectively improve the pervaporation separation performances.     

Pervaporation performance analysis and prediction—using a hybrid solution-diffusion and pore-flow model

A hybrid mathematic model for pervaporation is proposed which incorporates the concepts of solution-diffusion model and pore model. The model allows performance prediction as well as the establishment of the internal concentration and pressure profiles within the membrane. The model parameters specific to the particular membrane and mixture system are determined using liquid sorption and pervaporation experimental data. The model is experimentally examined using ethanol–water mixtures and poly(dimethyl siloxane)–poly(vinyldiene fluoride) (PDMS–PVDF) composite membranes. The characteristics of flux and separation factor predicted using the model are in fair agreement with the experimental data under various feed concentrations and downstream pressures for different membrane arrangements, including single-layer, reverse single-layer and double-layer PDMS–PVDF composite membranes. Internal profiles of pressure, concentration and component mole fraction can be established using the model. Concentration polarization phenomena for ethanol and water are located at membrane interfaces and vapor–liquid interfaces, respectively. Performances of several different membrane designs are compared using the model.     

Influence of operation parameters on the separation of mixtures by pervaporation and vapor permeation with inorganic membranes. Part 1: Dehydration of solvents

The separation performance of two different commercially available tubular inorganic membranes was studied for solvent dehydration. The separation layers consisted of A-type zeolite and microporous silica. The membrane characteristics were determined as function of operating conditions such as feed composition, temperature, and permeate pressure in pervaporation and vapor permeation. Among different membranes of the same batch, flux and selectivity were reproducible within 10%. The partial flux of water as the preferentially permeating component increases linearly with the water vapor pressure difference between feed and permeate and depends only marginally (viscosity influence) upon the properties of the organic component. The flux of the organic (retained) component is low and can best be described by assuming a substance and membrane specific permeance (flux over partial pressure difference) that is independent of composition. At very low water concentration in the feed one would expect a strong increase in permeability of the retained component through non-zeolite pores and larger silica pores as predicted by pure component measurements. However, this effect was not observed in mixtures within the concentration range studied here. A temperature rise improves flux rates exponentially while selectivity remains high. Thus, higher module cost in comparison to polymeric membranes can be compensated by reduced membrane area if a higher operating temperature can be chosen. Flux and selectivity decline as a function of permeate pressure with decreasing driving force. In vapor permeation with inorganic membranes superheating of the vaporous feed improves their performance while for polymeric materials a steep flux decline is observed. High flux and selectivity are obtained in the separation of water from alcohols. The normalized flux values of the A-type zeolite membrane are roughly 10 kg/m² h bar with a mixture selectivity of 2000 for methanol, 4000 for ethanol and 8000 for n-butanol. The average permeance of the amorphous silica membrane lies above 12 kg/m² h bar with mixture selectivity of 50 for methanol, 500 for ethanol and 2000 for n-butanol. The separation mechanism is mainly based on adsorption and diffusion enhanced by shape selectivity and size exclusion in some cases. The transport characteristics could be described with a simple transport model based on normalized permeate fluxes. With regard to the operation stability of the membranes, no deterioration of the performance was observed for the A-type zeolite in solvent dehydration or in separation of water from reaction mixtures. The silica membrane showed an initial conditioning effect involving a rearrangement of Si-OH groups with an increase in selectivity and decrease in flux of about 30%. After a few hours the performance stabilized and remained constant during further operation.     

CONCENTRATION POLARIZATION OCCURRING INSIDE THE MEMBRANE DURING STEADY STATE PERVAPORATION

Experimental profiles of a single penetrant (water) across the membrane have been established at different downstream pressures during steady state pervaporation. The profiles ofacetic acid-water binary penetrant system across the membrane were also measured at different downstream pressures, temperatures and compositions during steady state pervaporation. A stack of identical pre-characterized symmetric aromatic polyamide membranes was used for the profile study. The theoretical prediction of concentration polarization from mathematical equations has been confirmed by the experimental profile data for a binary penetrant system.     

甲醛－水渗透蒸发膜分离

制备了乙烯共聚醋酸乙烯酯复合膜和聚酰亚胺均相致密膜，研究了这两种膜与全氟磺酸型离子交换膜的渗透蒸发分离性能。实验测得最大分离系数α＝２７４，并发现料液的甲醛浓度增加，膜的分离系数提高。     

Dehydration of water/1-1-dimethylhydrazine mixtures by zeolite membranes

In this research, dehydration of water/1-1-dimethylhydrazine (UDMH) mixtures by zeolite NaA and hydroxy sodalite membranes has been investigated. Support of these membranes has been tubular mullites that have been made by extruding a mixture of about 67–75% kaolin clay and 33–25% distilled water using an extruder. Zeolite NaA and hydroxy sodalite membranes have been coated on the external surface of the porous supports by the hydrothermal synthesis.

UDMH/water mixtures have been separated at ambient temperature and pressure by pervaporation (PV) using these zeolite membranes. These membranes showed very high selectivity of water for all UDMH mixtures. For the UDMH/water mixtures, separation factor as high as 10 000 has been obtained for UDMH feed concentration of 2%. Total mass fluxes of 1.05–0.2 kg/(m² h) have been also obtained.     

Irradiation Grafted Polymeric Films. II. Gas Transport Properties of Hydrated Potassium Acrylate-Grafted Polyethylene Films

Abstract

The permeability constants of oxygen and carbon dioxide through hydrated potassium acrylate-grafted polyethylene films increase rapidly as the degree of hydration of the films increases above about 28 wt %. Below about 28 wt %, the carbon dioxide permeability constant increases with the degree of hydration. In the case of oxygen, the opposite is true.

The separation factor (CO₂/O₂) increases rapidly with film hydration up to about 28 wt %. Above this degree of hydration, the separation factor gradually approaches the value for pure water. An explanation for the results obtained is presented.     

Zeolite filled polyimide membranes for dehydration of isopropanol through pervaporation process

Six mixed matrix membranes (MMMs) were prepared using zeolites of 4A and ZSM-5 incorporated in polyimide of Matrimid 5218. Effects of filler type on membrane morphology and pervaporation performance of MMMs were investigated using isopropanol dehydration. In addition, effects of operating temperature (30, 40, 50, and 60 °C), feed water concentration (10, 20, 30, and 40 wt.%) and permeate side pressure (0 and 15 torr) on pervaporation performance were studied. Scanning electron microscopy (SEM) analysis showed there were good adhesion between the fillers and the polymer matrix. Zeolite 4A has a better contact with the polymer phase and thereby nearly no void is formed in the MMM structure. Pervaporation were performed based on L16 array of Taguchi method for design of experiments. The results showed that the best separation condition is achieved at temperature, feed water concentration, and permeate pressure of 30 °C, 10 wt.% water and 0 torr, respectively. Selectivities of zeolites 4A and ZSM-5 filled MMMs were calculated as 8991 and 3904 compared with 1276 measured for the neat Matrimid 5218 membrane. Permeation rates of the zeolite 4A and ZSM-5 filled MMMs and the neat polymeric membrane were found to be 0.018, 0.016, and 0.013 kg/m² h, respectively.     

Influence of molecular weight of polydimethylsiloxane precursors and crosslinking content on degree of ethanol swelling of crosslinked networks

Using PDMS (polydimethylsiloxane) as a basic polymeric matrix to the preparation of ethanol-permselective pervaporation membranes is a vibrant field of research. In this paper, a detailed study of the effects of the molecular weight of PDMS precursors and the content of the TEOS (tetraethyl orthosilicate) crosslinker on the degree of swelling in ethanol and ethanol contact angle is reported. Five PDMS precursors with molecular weights of 26.6 K, 35.5 K, 50.2 K, 71.7 K, and 110.4 K, and five crosslinking contents (1 wt%, 2 wt%, 5 wt%, 10 wt%, and 15 wt%) were chosen to prepare twenty-five PDMS networks. Considering only the maximum tensile strength of the networks, the optimum molecular weight of the precursor was found to be 35.5 K and the optimum crosslinker content was 5 wt%. The average Young’s modulus of the PDMS network prepared under these conditions reached 0.63 MPa after using toluene to extract the network. Some uncrosslinked precursors always occur in the networks, and have some influence on the molecular weight of the precursors and the crosslinker content that is used. It was found that the content of the uncrosslinked precursors has direct effect on the contact angle of ethanol sessile drops at the surface of the extracted PDMS networks, and higher extraction corresponded to a smaller ethanol contact angle. A combined parameter (S), defined as the quotient of the extraction amount (A_E) and the tensile elastic modulus (E_Y), gives a good linear relationship with the increase in weight of networks swelled in ethanol. This means that the degree of equilibrium swelling of the networks is simultaneously strongly influenced by the tensile modulus and the content of the uncrosslinked precursors.